In multipath transmission systems, such as those which utilize troposcatter or high frequency communication links, for example, the transmitted signal is conveyed through a medium along a plurality of paths of differing lengths so that a plurality of signals, each representing the transmitted signal but having varying energy contents, are received at the receiver at different times depending on the length of each particular transmission path in the medium. In some applications redundant, or diversity, channels each containing a plurality of differing path lengths are used to decrease the vulnerability to fading effects.
A general discussion of such communication systems and the problems associated therewith can be found in the paper "Fading Channel Communications" by Peter Monsen, published in IEEE Communications Magazine, January 1980, pages 16-25. Such paper discusses general approaches to receivers for use in such circumstances and particularly discusses various prior art approaches to adaptive receiver structures, particularly for reducing the effect of intersymbol interference.
A more specific discussion of a particular communications receiver for processing received diversity signals and eliminating intersymbol interference is described in U.S. Pat. No. 3,879,664, issued on Apr. 22, 1975 to Peter Monsen. In such system the receiver utilizes a forward adaptive filter equalizer, having a plurality of weighting sections, in each of the receiver diversity channels for processing each of the received diversity signals. The combined weighted output signal is used to reconstruct the transmitted data and to generate an error signal. The error signal is used to derive appropriate adaptive weighting signals for use in the processing of the received diversity signals at each of the forward filter equalizers.
The forward filter equalizers generally use tapped delay lines to provide for appropriate matched filtering of the received signals. The delay lines are arranged so as to have a sufficient number of taps to capture as much of the useful energy of the pulses in the currently received signals as possible and to tend to cancel, or to significantly reduce, the cross-talk from succeeding pulses (i.e., to reduce future intersymbol interference). The system may further utilize a backward filter, also known as a decision feedback equalizer circuit, which suitably processes the reconstructed data to form a cancellation signal which is effectively used to eliminate past intersymbol interference (i.e., cross talk from preceding pulses). The same error signal is also used to derive appropriate weighting signals in the backward filter equalizer.
In a typical practical embodiment of such a system, the tapped delay line in each forward filter equalizer uses multiple taps spaced at less than the Nyquist interval in order to accomplish the two functions of signal energy capture and of future intersymbol interference cancellation, as described. for example, in U.S. Pat. No. 3,633,107, issued on Jan. 4, 1972 to D. M. Brady. It has been subsequently suggested that better performance can be achieved by treating the energy capture problem separately from the intersymbol interference problem. Such systems are described in U.S. Pat. No. 4,271,525, issued on June 2, 1981 to Kojico Watanabe, and in U.S. Pat. No. 4,328,585, issued on May 4, 1982 to Peter Monsen. In such systems, the matched filtering process for effectively capturing signal energy is separated from the future intersymbol interference correction process by utilizing independent circuits for performing each function.
As described in U.S. Pat. No. 4,271,525, for example, a diversity combining filter utilizing a plurality of separate adaptive matched filters using multiple tapped delay lines within each diversity channel performs the matched filtering process by producing a time reverse of the impulse response of the channel yielding the input signals thereto, the outputs of said matched filters being appropriately combined, as by summing, to produce a combined matched filter output. U.S. Pat. No. 4,328,585 improves on this concept by using a lattice filter to speed up adaptation of the intersymbol interference cancellation function. Thus, a single suitable intersymbol interference correction equalizer circuit utilizing a single multiple tapped delay line is used to respond to the combined matched filter output for performing intersymbol interference elimination separately from the diversity combining matched filter process of the system.
In implementing practical versions of such systems, matched filters are used for each diversity branch of the diversity system each matched filter using a delay line having multiple taps, while the single intersymbol interference correction equalizer also uses a single multiple tapped delay line. The diversity signals at each tap are weighted and combined, all of the weighted signals at all of the taps then being further combined at the output of the delay line to produce an output signal for supplying to the intersymbol interference correction equalizer circuit.
Relatively complicated weighting circuitry is utilized at the matched filters for providing the desired weights for each of the diversity signals which are combined to supply the weighted signals at each of the delay line taps. Relatively precise weight calculations are used to provide the desired improved performance characteristics.
In some applications, however, it is desirable to simplify even further the complicated weighting techniques used in such systems while still maintaining reasonable and aceptable performance characteristics.